1. Technical Field
The present disclosure relates to a control device for a DC-DC converter, in particular for a flyback converter.
2. Description of the Related Art
It is important for many circuit applications for converters to obtain an excellent performance during the operation at average or maximum load and also a good performance during the operation with low load, also less than a watt.
Among the various circuit topologies implementable capable of meeting the aforesaid minimum efficiency limits at average or maximum load, the resonant topologies or soft-switching of half-bridge type are very common, like the converter with flyback-type operation shown in FIG. 1, which comprises two electronic switches S1, S2 arranged in series, connected between the DC supply voltage Vin and the ground GND.
The load is generally connected by means of a transformer 10 having the purpose of isolating the output ground from the input and moreover of permitting a control between the output voltage Vout and the input voltage Vin. The converter comprises a capacitor Cr and an inductance L1 arranged in series to the transformer 10 and a further circuit block, comprising a diode D1 and a capacitor Cout, interposed between the transformer 10 and the load, adapted to rectify and filter the voltage from the secondary of the transformer to make it compatible with the features required by the load.
The topologies of half-bridge soft-switching converters comprise a push-pull control of the electronic switches S1-S2, with a short delay period interposed between the turn off of one switch and the turn on of the other one, called dead time, during which there is no conduction of both. When the switch S1 is turned on, there is the temporary connection of the transformer to the input voltage Vin with the consequent storage of part of the energy in the magnetization inductance Lm of the transformer by means of the circulation of a current I1. During the successive turn off step of the same switch S1 and consequent turn on of the other switch S2, there is the closing of a loop comprising the transformer 10, the capacitor Cr and the inductance L1 and a resonance may occur between the elements; there is a circulation of current I2 in said step. The transfer of energy to the load may occur during the turn on step of the switch S1, i.e., during the storing step of the energy, and in this case a forward-type converter will be obtained, or during the turn on step of the switch S2, thus obtaining flyback converter like the one shown in FIG. 1.
The control type of these typologies is generally at fixed or variable frequency and variable duty cycle. The peculiarity of the switching during turn on of the switch elements having zero voltage (so-called ZVS operation) also permits to obtain very high circuit performances at full load. However, by decreasing the load to a small fraction of the maximum value, the overall efficiency of the circuit decreases significantly due to the predominant leakages of the circuit switching elements which are directly proportional to the operating frequency of the circuit.
Various techniques have been implemented to decrease such leakages and to try to minimize them; most of such techniques are about decreasing the operating frequency of the circuit and are called “frequency foldback” or “frequency shifting”.
With such a technique the operating frequency is decreased as the circuit load decreases, thus contributing to decreasing the leakages and hence to increasing the efficiency at decreased loads. Such a decrease may occur both suddenly, i.e., at a certain threshold the circuit is made to operate at a lower frequency, or the frequency may be decreased continuously, proportionally with the value of the load.
A highly widespread type of control for power converters is called “current mode”. In this type of control, the instantaneous current flowing in the circuit during the step in which the generator supplies power to the circuit is compared with the output signal of an error amplifier adapted to amplify the error between a voltage proportional to the output voltage Vout and a reference voltage Vref. The intersection of the output signal of the error amplifier and of the instantaneous current determines the duration of the turn on time of the switch S1 which connects the supply voltage Vin to the circuit transferring the power to the load. The output signal of the error amplifier has an amplitude equal to the amplitude of the current signal for the circuit gain; given that the current flowing through the circuit during the turn on step of the switch S1 is proportional to the input power, the result is that the output signal of the error amplifier is proportional to the input power of the converter itself and therefore, without considering the variation due to the efficiency of the circuit, to the output power supplied to the load, without depending on the input voltage.
In the half-bridge ZVS flyback in FIG. 1, the switches S1 and S2 are push-pull controlled. In this case too, during the turn on period of the switch S1 there is an accumulation of energy in the magnetization inductance Lm and in the dispersion inductance L1 of the transformer. During the successive turn off step of the switch S1, the current is not cancelled but will flow through the switch S2 again which S2 will be turned on during such a step. During such a time period the transfer of part of the accumulated energy to the secondary and thus to the load will be obtained, while part of the energy not transferred will result in a re-circulating current through the primary. Hence the current Isense flowing through the primary, detected by the sense resistance Rs, will have a waveform given by the average current supplied at the output by means of the mutual inductance, to which a ramp overlaps due to the current flowing through the magnetization inductance Lm of the transformer. Hence, due to these two components, the result will be, also under conditions of zero load, that the signal detected on the sense resistance Rs will have a minimum amplitude depending on the current circulating through the magnetization inductance Lm of the transformer.
For such a reason the output of the error amplifier in a current mode control system under conditions of low load is no longer proportional to the load itself due to the current circulating through the magnetization inductance of the transformer. Hence there will be a minimum voltage at the output of the error amplifier which will depend on the parameters of the power circuit, i.e., information proportional to the load at the output of the error amplifier is not available.
Furthermore, the current circulating the magnetization inductance of the transformer prevents using the technique of decreasing the switching frequency mentioned above. Indeed, as the feedback keeps the duty cycle constant, a decrease of the frequency would determine an increase of the charge time of the magnetization inductance. Thus an increase of the primary current peak and some related leakages would be obtained and the output of the error amplifier would increase. Thereby each decrease of the frequency proportional to the output of the error amplifier (as is commonly done in various devices) would lead to a consequent re-increase of the switching frequency. Increasing the switching frequency would re-lower the output level of the error amplifier which would be interpreted by the control circuitry as a low load operating condition. This would generate a condition of operating instability for the circuit.